The invention is based on a fuel injection device for internal combustion engines, in particular a unit fuel injector, as defined hereinafter.
Generic Injection devices, particularly when embodied as unit fuel injectors are used particularly where high injection pressures and the simplest possible adaptability of the course of injection to the engine combustion course requirements, dictated by various parameters, are needed. Existing requirements involve not only the compressibility of the fuel, which has a more pronounced effect on the injection principle because of the high injection pressure, and the resultant influences on the course of injection above all they involve great freedom in adaptability of the onset and end of injection. Normally in these fuel injection devices the drive cam is disposed on the engine camshaft, on the one hand to bring the high drive forces to bear with as little loss as possible and on the other hand to obtain control of the injection device as a function of the rotational angle in a manner that functions as synchronously as possible with the valve control. This rotational angle-dependent control has superimposed on it the flexible timing control of the fuel metering device, which is preferably in the form of a magnetic valve, to enable making the appropriate closed- and open-loop control interventions via an electronic control unit. Preferably the trailing flank of the drive cam (intake stroke) is embodied as relatively long; it encompasses a wide rotational angle, so that a relatively long period of time is available for control via the fuel metering device during the intake stroke. A number of variants of such injection devices are known.
In a known unit fuel injector of the type set forth by (German Offenlegungsschrift 37 00 352), the pump piston serves as the control device; via an annular groove disposed on its jacket face, during a metering segment of the intake stroke of the pump piston, the metering conduit communicates with the pressure chamber inflow line, so that the desired injection quantity, determined by the fuel metering device, can be delivered to the pressure chamber. After that, the metering conduit is blocked via an annular shaft of the pump piston, so that then in the terminal position of the intake stroke, in accordance with a detent portion of the cam, the metering conduit can be made to communicate with the pump chamber and introduce into it the quantity defined by the fuel metering device, which determines the supply onset. The free piston dividing the pump chamber and pressure chamber assumes any arbitrary intermediate position at this time, because voids are created during the intake stroke in both the pressure chamber and the pump chamber, to the extent that the intake stroke of the pump piston is greater than the fuel quantities necessary to fill the two chambers. The void formation is typically at an extreme, particularly in the pressure chamber, at the onset of fuel metering into the pump chamber.
The largely uncontrolled "floating" of the free piston, at high rpm, or in other words high piston speeds, can cause deviations in the metering of the fuel quantity into the pressure chamber or the pump chamber, because the quantity of fuel flowing through the metering device is determined both by the cross section and by the pressure of the fuel source, and because of mass acceleration or inertia the mass of the free piston can have an rpm-dependent effect. A further factor is that these rpm-dependent mass factors can cause the void in the pressure chamber to be larger than in the pump chamber, so that when the fuel quantity determining the supply onset is metered into the pump chamber, there may not be enough room available, or the pumping might have to be performed counter to the pumping action of the free piston dictated by the acceleration of the free piston.
For this reason, in another, quite similar fuel injection device (German Offenlegungsschrift 37 00 359), a spring-loaded intermediate piston is provided instead of the free piston; via the fuel metering device, the fuel quantity determining the supply onset is first metered into the pump chamber, and as the intake stroke continues, after the reversal of either the pump piston or the intermediate piston, the quantity of fuel to be injected is metered into the pressure chamber. During the fuel metering into the pump chamber, the intermediate piston is largely kept in its outset position by the pressure of the fuel flowing into the pump chamber, or in other words by the pressure of the fuel source, counter to the force of the spring, and then after the termination of fuel metering into the pump chamber (or in other words after the termination of the metering of the fuel quantity determining the supply onset) is displaced by the spring in accordance with the continued intake stroke of the pump piston. Once the maximum stroke has been executed, this intermediate piston strikes a stop. In this terminal position, the pressure chamber inflow line is opened by the intermediate piston, while the pump piston continues its intake stroke to the end. In this continued intake stroke, a void is initially created in the pump chamber--on account of the stoppage of the intermediate piston because of the stop--and second, the fuel that is to be injected is now metered into the void, created by the spring, in the pressure chamber. Although the void at the beginning of metering of the quantity of fuel to be injected is of equal size at each injection cycle, effects of the metering that determines the supply onset do exist, because the intake stroke of the intermediate piston always begins whenever the metering into the pump chamber that determines the supply onset has ended. The difference in pressure between the pressure of the fuel source and the negative pressure in the pressure chamber varies in the course of the metering of fuel into the pressure chamber, because as it fills, the negative pressure decreases correspondingly, so that this also has an effect on the timing control or in other words on the metering defined over time. To reduce this influence, a throttle is provided in this pressure chamber inflow line; the throttle effects a certain backup of the fuel source pressure compared with the pressure chamber negative pressure and thus effects a certain decoupling. Not only is there the disadvantage that metering is done into a void of variable negative pressure, but the total stroke of the pump piston is not favorably exploited, either. A further disadvantage is that such a spring is structurally complicated and particularly difficult to manufacture, which makes it expensive.